As a conventional mobile communication method using an internet protocol, the one using Mobile IPv6 which is disclosed in “Mobility Support in IPv6”, IETF RFC3775 written by David B. Johnson et al. is known.
The operation of the conventional Mobile IPv6 will be explained with reference to a sequence diagram of FIG. 7. Note that black circles in the drawing indicate routing nodes.
First of all, a mobile communication apparatus MN holds a home address (hereinafter, referred to as HoA) which will not change with movement, and is connected to an access router (hereinafter, referred to as OAR). It receives data packets from a correspondent node (hereinafter, referred to as CN) through the OAR (step S700).
The mobile communication apparatus moves to another access router (hereinafter, referred to as NAR) from this state, and generates a care-of address (hereinafter, referred to as CoA) based on a subnet prefix included in a router advertisement (hereinafter, referred to as RA) message (step S701) received from the NAR after a handover in a data link layer is finished (step S702).
Next, the mobile communication apparatus MN sends a binding update (hereinafter, referred to as BU) message including HoA and CoA to a home agent HA (step S703).
The home agent HA, after receiving the BU message from the mobile communication apparatus MN, in the case that there exists an entry which corresponds to the HoA included in the received BU message in a binding cache stored by the home agent HA itself, updates the entry. In the case that there does not exist a corresponding entry, a new entry is generated. After that, the home agent HA sends to the mobile communication apparatus MN a binding acknowledgement (hereinafter, referred to as BA) message indicating that a registration is completed (step S704).
Next, the mobile communication apparatus MN confirms that the registration is completed by receiving the BA message. According to the above, packets sent addressed to the HoA of the mobile communication apparatus MN are intercepted by the home agent HA and tunneled to the CoA of the mobile communication apparatus MN.
However, in these standard operations of Mobile IPv6, all packets addressed to the HoA of the mobile communication apparatus MN are routed via the home agent HA; therefore, the route will be redundant.
Therefore, in Mobile IPv6, a route optimization procedure which allows packets to be directly transmitted to the mobile communication apparatus MN without routing via the home agent HA is prescribed as an optional feature. In the route optimization procedure, the mobile communication apparatus MN, after the registration to the home agent HA as the standard operation has been completed, sends the BU message including the HoA and CoA to the correspondent node CN with which the apparatus currently communicates (step S706). Or, when a received packet is tunneled by the home agent HA, the BU message including HoA and CoA is sent to the correspondent node CN which is the transmission origin and is the node with which communication is to be made.
Next, the correspondent node CN which has received the BU message from the mobile communication apparatus MN updates or generates the entry concerning the HoA of the mobile communication apparatus MN in the binding cache in the same way as the home agent HA. In the case of performing a registration processing to the correspondent node CN, a procedure for securing security (step S705) might be performed.
According to the above operation, the correspondent node CN can forward a packet directly to the mobile communication apparatus MN without sending the packet to the corresponding care-of address and thus being routed via the home agent HA, because there exists a destination address of the transmission packet in the binding cache (step S707).
According to the route optimization procedure, packet transmission delay can be made small, which is effective when performing close-to-real-time communication. For example, when an application to be used requires the real-time communication, there is a method of decreasing the packet delay by executing the route optimization procedure as disclosed in JP-A-2001-144811.
However, in the conventional structure, when the mobile communication apparatus moves to another access router while communicating with the correspondent node with the route optimization performed, the apparatus starts a location registration to the correspondent node after the location registration with respect to the home agent has been completed, and therefore, packets transmitted to the care-of address of the mobile communication apparatus during the registration are transmitted to the care-of address before the movement, so that there is a problem that packet loss occurs.